Oligonucleotide analogs containing nuclease resistant (3'-5') methyl phosphonate internucleoside linkages (d-NOP(O)(CH3)ON) will be studied for their ability to serve as substrates for enzymes involved in DNA synthesis and repair. Analogs with base sequences complementary to selected polynucleotides and to regions of phi X 174 DNA and Rous sarcoma virus RNA will be prepared. The interaction of these analogs with their complementary polynucleotides will be evaluated. The analogs will be tested for their ability to initiate in vitro DNA synthesis at specific regions on their complementary templates catalyzed by DNA polymerase and reverse transcriptase. These analogs will also be tested as substrates for polynucleotide kinase, DNA ligase, and RNA ligase. In addition, oligonucleotide duplexes having regions which contain phosphodiester and methyl phosphonate linkages will be prepared. These duplexes will be tested as substrates for the joining enzyme, DNA ligase. The resulting sealed duplexes which will contain the Hin d III restriction enzyme site, A precipitates AGCTT, will be tested as substrates for Hin d III. The above experiments will attempt to assess the effects of backbone modification on celluar DNA synthesis at the substrate/initiator and template levels. In addition, these experiments will assess some of the parameters involved in protein-nucleic acid interactions.